Peroxisome proliferators are a structurally diverse group of compounds which, when administered to rodents, elicit dramatic increases in the size and number of hepatic and renal peroxisomes, as well as concomitant increases in the capacity of peroxisomes to metabolize fatty acids via increased expression of the enzymes required for the .beta.-oxidation cycle (Lazarow and Fujiki, Ann. Rev. Cell Biol. 1:489-530 (1985); Vamecq and Draye, Essays Biochem. 24:1115-225 (1989); and Nelali et al., Cancer Res. 48:5316-5324 (1988)). Chemicals included in this group are the fibrate class of hypolipidermic drugs, herbicides, and phthalate plasticizers (Reddy and Lalwani, Crit. Rev. Toxicol. 12:1-58 (1983)). Peroxisome proliferation can also be elicited by dietary or physiological factors such as a high-fat diet and cold acclimatization.
Insight into the mechanism whereby peroxisome proliferators exert their pleiotropic effects was provided by the identification of a member of the nuclear hormone receptor superfamily activated by these chemicals (Isseman and Green, Nature 347-645-650 (1990)). This receptor, termed peroxisome proliferator activated receptor alpha (PPAR.alpha.), was subsequently shown to be activated by a variety of medium and long-chain fatty acids and to stimulate expression of the genes encoding rat acyl-CoA oxidase and hydratase-dehydrogenase (enzymes required for peroxisomal .beta.-oxidation), as well as rabbit cytochrome P450 4A6, a fatty acid .omega.-hydroxylase (Gottlicher et al., Proc. Natl. Acad. Sci. USA 89:4653-4657 (1992); Tugwood et al., EMPO J. 11:433-439 (1992); Bardot et al., Biochem. Biophys. Res. Comm. 192:37-45 (1993); Muerhoff et al., J. Biol. Chem. 267:19051-19053 (1992); and Marcus et al., Proc. Natl. Acad. Sci. USA 90(12):5723-5727 (1993). The foregoing references support a physiological role for PPAR.alpha. in the regulation of lipid metabolism. PPAR.alpha. activates transcription by binding to DNA sequence elements, termed peroxisome proliferator response elements (PPRE), as a heterodimer with the retinoid X receptor. The retinoid X receptor is activated by 9-cis retinoic acid (see Kliewer et al., Nature 358:771-774 (1992), Gearing et al., Proc. Natl. Acad. Sci. USA 90:1440-1444 (1993), Keller et al., Proc. Natl. Acad. Sci. USA 90:2160-2164 (1993), Heyman et al., Cell 68:397-406 (1992), and Levin et al., Nature 355:359-361 (1992)). Since the PPAR.alpha.-RXR complex can be activated by peroxisome proliferators and/or 9-cis retinoic acid, the retinoid and fatty acid signaling pathways are seen to converge in modulating lipid metabolism.